Plastic tubings are extensively employed in the medical field, particularly for patient analysis and treatment procedures. Various plastics and combinations thereof are used, depending upon the specific properties the intended application demands. The selection of desired plastic materials is further limited by the use of the tubing in the in vivo treatment of human patients, as the tubing may be used in the administration of intravenous fluids or itself may be introduced into the body. Thus numerous factors must be considered in ascertaining which materials to choose.
Polyvinyl chloride (PVC) is the standard material used, made with suitable plasticizers necessary to enhance flexibility and other properties. However, such plasticizers or similar additives have a tendency to migrate, causing hazardous contamination with the fluid being administered. The contamination becomes more serious where the fluid is introduced into the body, as contamination of the blood may result, Moreover, plasticized PVC tubings have been shown to absorb nitroglycerin and insulin, and are thus unsatisfactory for the administration of these medicines. Much effort has been directed towards finding an alternative that does not suffer from the limitations of the plasticized PVC tubing. Polyurethane has been used, as in U.S. Pat. No. 4,211,741 to Ostoich. Because it is a relatively soft, flexible plastic even without additives, additives are not necessary, their absence thereby minimizing migration possibilities. In addition, it exhibits good fluid flow characteristics. However, the high cost of polyurethane has limited its use to only extraordinary applications. Some grades of ethylene-vinyl acetate copolymer (EVA) are currently being used as an outer layer, together with low-density polyethylene (LDPE) as an inner layer. Although this composite exhibits excellent peel strength, it lacks flexibility, clarity, and is easily scuffed or roughened. In addition, it cannot be solvent bonded. Since the tubing is the connecting link between a reservoir of fluid (nitroglycerin, insulin, etc.) and the patient, the method of assembly is an important consideration. Where, as here, solvent bonding cannot be utilized, an expensive, less reliable mechanical means of assembly are required, whereby a PVC layer must be pressure fit over the EVA-LDPE tubing to utilize the solvent bondable characteristics of PVC. For these reasons, the EVA-LDPE product has proven to be unsatisfactory.
The ideal product would have a thick, flexible outer layer comprised of PVC and a thin layer of LDPE, thus exploiting their respective advantages through strategic placement. Under normal extrusion conditions of temperature and pressure, however, PVC and LDPE will not bond together. Consequently, undesirable layer separation occurs. A third or "tie layer" is therefore required which will bond to both the outer PVC layer and the inner LDPE layer.
It is therefore, an object of the present invention to provide a coextruded, tri-layered, solvent bondable, soft, flexible plastic tubing which can be utilized in the administration of nitroglycerin and insulin.
Other and related objects and advantages will become evident from the following specification and claims.